Shaft seals have been known for a long time and are used in steam turbines to seal between rotating components. One common seal is a retractable seal that comprises an arcuate segment with a sealing surface on which labyrinth sealing elements are located and a necked-in portion on a pressure side of the seal segment. The necked-in portion is configured to fit into a T-slot configured in the stationary component. The segments of a segment row are serially disposed in the circumferential direction within their T-slot and arranged so that each segment is resiliently yielding in the radial direction, substantially independently of the adjacent segments, so that when touched under the action of the applied radial thrust it deflects radially and returns into its starting position when the thrust is relieved. This resilient yield is obtained by means of helical springs or leaf springs which are accommodated in recesses of the segments and whose action in the operating state is increased by slots at the rear sides of the segments which allow steam of higher pressure to pass to the rear side of the segments so that these are additionally radial thrust towards the interior.
The necked-in portion further includes an axial facing surface. Axial pressure drop across the gland gives a thrust, and this force is transferred to the stationary component through the axial face. If the gland is required to move radially then friction resulting from the axial thrust can be high. The main benefit of the design is to reduce the axial thrust, thereby reducing the friction on the axial face. This ensures that when the gland is required to move radially, it will be more responsive to the radial forces acting upon it. It is not desirable to remove all friction, as this could result in a gland that “chatters”, i.e. small, high frequency, varying forces causing small movements of the gland. A moderate amount of friction will damp these small movements. Therefore, the design needs to control variables that affect friction.